It is conventional to correct a detection error of a rotation angle sensor caused by positioning error in attaching the rotation angle sensor to a multi-phase rotary machine. According to JP 2001-128484A (patent document 1), a positioning error in attaching a resolver to a three-phase motor is detected by comparing an actual motor position detected by the resolver with a theoretical position, at which the motor stops when two phases of three phases of the motor are shorted. A correction value is then calculated based on a comparison result. This correction value is stored in a memory section and used to correct a detected motor position.
According to patent document 1,it is assumed that the correction value need not possibly be changed after the correction value is calculated once at the time of manufacture of a motor, unless the resolver and the motor are detached together from a vehicle after having been mounted once on the vehicle. In case of a motor, which is used in an electric power steering system of a vehicle, for example, a motor body and an electronic control unit (ECU), which controls a motor body in accordance with a motor rotation angle and the like, may be assembled detachably from each other. A rotation angle sensor may be formed, for example, by a combination of a magnet provided at the motor body side and a magnetic detection device provided at the ECU side. According to this sensor configuration, a positioning error will arise between the ECU and the motor when only the ECU is detached and replaced with the motor body being fixedly attached to the vehicle. In this instance, a correction value need be calculated again.
In patent document 1,it is assumed that the shorting of two phases among three phases is performed for each part unit while maintaining the rotary shaft in a free state. As a result, it is not possible under a state that the motor body is mounted on a certain structure with its rotary shaft being coupled to a load. It is therefore required to demount the motor once from the certain structure or replace both the motor and the ECU together for a sole purpose of calculating the correction value. This is not efficient.